The present invention generally relates to semiconductors, and more specifically, to passivation processes directed to photovoltaic devices.
Organic materials are of interest for photovoltaic applications for various reasons. For example, organic materials are relatively low in cost and can be processed over large surface areas, e.g., on flexible low-cost substrates. However, the efficiency of photovoltaic devices with such organic materials is lower than devices with inorganic materials. The smaller diffusion length of minority carriers in organic materials, compared to inorganic materials, can decrease efficiency.
Accordingly, heterojunction devices, such as in photovoltaic devices, that include an inorganic substrate (an absorption layer), for example silicon, with an organic contact are particularly attractive. Such devices combine the organic material's low-temperature, large-area processing capability with the inorganic material's large diffusion length, which substantially eliminates excessive recombination in the absorption layer.
In heterojunction devices, the dangling bonds at the surface of the inorganic material are passivated to minimize recombination loss at the organic/inorganic interface. Wide band gap materials, for example, PQ (9,10-phenanthrenequinone), have been used to passivate the surface of silicon (Si). Referring to FIG. 1, the lowest unoccupied molecular orbital (LUMO)/conduction band (Ec) offset in PQ repels minority electrons, which is favorable for reducing dark current. However, because of the highest occupied molecular orbital (HOMO)/valence band (Ev) offset at the Si/PQ interface, the potential barrier may reduce the collection of majority holes at the emitter junction and drastically reduce photocurrent. These effects hamper the use of mature hole-transport organic materials, such as pentacene (also shown in FIG. 1), to form a high-performance heterojunction solar cell devices on n-type silicon.